1. Field of the Invention
The present invention relates to the etherification of isoolefins, particularly C.sub.4 and C.sub.5 isoolefins, with an alcohol such as methanol to produce the corresponding tertiary ether. More particularly the invention relates to a process wherein a catalytic distillation process is used in the process and wherein the overheads from the distillation column reactor contain significant amounts of the alcohol and isoolefin. More particularly the invention relates to a process wherein the overheads are subjected to further reaction and the effluent from the reactor is fed back to the distillation column as reflux or stripper feed or a combination of both.
2. Related Information
The reaction of an alcohol and an olefin and concurrent separation of the reactants from the reaction products by fractional distillation has been practiced for some time. The process is variously described in U.S. Pat. Nos. 4,232,177; 4,307,254; 4,336,407; 4,504,687; 4,987,807; and 5,118,873, all commonly assigned herewith.
Briefly, the alcohol and isoolefin are fed to a distillation column reactor having a distillation reaction zone containing a suitable catalyst, such as an acid cation exchange resin, in the form of catalytic distillation structure, and also having a distillation zone containing inert distillation structure. As embodied in the etherification of iC.sub.4.sup.= 's and/or iC.sub.5.sup.= 's the olefin and an excess of methanol are first fed to a fixed bed reactor wherein most of the olefin is reacted to form the corresponding ether, methyl tertiary butyl ether (MTBE) or tertiary amyl methyl ether (TAME). The fixed bed reactor is operated at a given pressure such that the reaction mixture is at the boiling point, thereby removing the exothermic heat of reaction by vaporization of the mixture. The fixed bed reactor and process are described more completely in U.S. Pat. No. 4,950,803 which is hereby incorporated by reference.
The effluent from the fixed bed reactor is then fed to the distillation column reactor wherein the remainder of the iC.sub.4.sup.= 's or iC.sub.5.sup.= 's are usually converted to the ether and the methanol is separated from the ether which is withdrawn as bottoms. The C.sub.4 or C.sub.5 olefin stream generally contains only about 10 to 60 percent olefin, the remainder being inerts which are removed in the overheads from the distillation column reactor.
In some cases the distillation column reactor may be operated such that complete reaction of the isoolefin is not achieved for a particular reason and therefore there may be significant isoolefin in the overheads, that is, from 1 to 15 wt %, along with unreacted methanol.